The present invention relates to a dual line glow plug that is mounted in a diesel engine for preheating a precombustion chamber in the engine head.
Diesel engines are usually slow in starting up at low temperatures. In order to solve this problem, a glow plug is attached to a precombustion chamber in the engine head and an electric current is applied to the plug so that it becomes sufficiently red hot to cause partial burning of the fuel being injected into the chamber. Today, the use of such a glow plug is not limited to this purpose of preheating alone and it is more often subjected to prolonged use as an afterglow for ensuring stable combustion after engine start-up.
A conventional glow plug of the type described above is a ceramic glow plug which uses a ceramic heater as the incandescent body. This heater is comprised of an incandescent wire of a refractory or high melting point metal which is embedded within a ceramic sinter, and the heater provides a rapid increase in temperature. A commonly used ceramic glow plug is of the single line type, wherein one end of the incandescent wire in the heater is connected to the central electrode that is insulated from the metal fitting and provides a positive electrode terminal, whereas the other end of the wire is electrically connected to the metal fitting to provide a grounded electrode.
With a multi-cylinder engine, a single-line glow plug is attached to each cylinder, thus providing a parallel array of glow plugs with respect to the power supply. The individual glow plugs must be properly controlled depending upon the heating conditions that are necessitated both at engine start-up and during the ensuing afterglow. To this end, an external circuit is necessary for controlling the impressed current without changing the source voltage but by simply changing the parallel glow plug array to a series connection or vice versa. In order to operate the external circuit, terminal electrodes connected to it become necessary and this in turn neccessitates a dual line glow plug. Considerable efforts have therefore been made to develop a commercial dual line glow plug and to incorporate various improvements in it.
A conventional dual line glow plug using a ceramic heater as the incandescent body is described in Published Japanese Utility Model Application No. 119045/1983 and is shown cross-sectionally in FIG. 1. A ceramic heater indicated at 1 is comprised of an incandescent wire 2 such as one made of a refractory metal which is embedded within a ceramic (e.g., Si.sub.3 N.sub.4) powder and sintered thereafter. The heater 1 is inserted into a metal fitting 3 until a shoulder 1a formed around the middle of the heater contacts and becomes firmly secured to a step 3a forming a constricted portion on the inner surface close to the bottom of the metal fitting 3. One end 2a of the incandescent wire 2 is connected to a central electrode 4 at the top of the heater 1 which provides one electrode terminal. The other end 2b of the wire is connected to an electrode tube 5 slipped over the heater 1 at its top end. The electrode tube 5 is lined with a heat-resistive insulator 6 so that the central electrode 4 is fitted tight in the tube 5. The tube 5 projects from the top of the metal fitting 3 and is insulated therefrom by an insulator 7. A terminal plate 8 is secured to the tube 5 for providing the other terminal electrode.
However, the conventional dual line ceramic glow plug shown above has the disadvantage of a complex shape in that a step must be formed on both the outer surface of the ceramic heater 1 and the inner surface of the metal fitting 3. This complexity in geometry increases the cost of the plug and may often cause the breakage of the ceramic heater being assembled.
Another conventional type of dual line ceramic glow plug is described in U.S. Pat. No. 2,030,937. In accordance with the invention shown in this patent, a ceramic heater is provided with a flange that is formed on the periphery of the top end and is brought into engagement with a step formed on the inner surface of a metal fitting. Alternatively, the flange is brought into engagement with a step formed on the inner surface of the bottom of a metal element and the periphery at the bottom of the metal element is pressed inwardly so as to secure the heater to that element. However, if this ceramic heater is mounted in a diesel engine, the heater will be unable to withstand hot combustion gases, high pressure or external impact, thus causing a gas leak or an increased contact resistance, and even a broken ceramic heater.